This invention relates to a process and apparatus for the production of pure hydrogen by steam reforming, and to the use of the hydrogen in a zero emission hybrid power system incorporating a fuel cell. The process integrates the steam reforming and shift reaction to produce pure hydrogen with minimal production of CO and virtually no CO in the hydrogen stream, provides for CO2 capture for sequestration, employs a steam reforming membrane reactor, and is powered by flameless distributed combustion that provides great improvements in heat exchange efficiency.
The production of electrical power in the most efficient manner with minimization of waste is the focus of much research. It would be desirable to improve efficiency in the production of electricity, separate and use by-product CO2 in other processes, and produce minimal NOx. The wide availability of natural gas with the highest H:C ratio (4:1) of any fossil fuel makes it a prime candidate for electricity production with minimum CO2 emissions.
In the production of electricity by conventional means only about 35% of the hydrocarbon fuel is converted to electricity and approximately 5% of that is lost over power lines. Even with modern turbines the efficiency is about 45%. In the case of the additional production of electricity by a xe2x80x9cbottom cyclexe2x80x9d where high temperature exhaust is used to boil water and produce more electricity, the combined efficiency is only about 60% in the lab. In addition, though about 3-5% m CO2is produced as exhaust from turbines, it is very difficult and expensive to capture due to the low concentration in the exhaust streams.
Electricity can be produced in fuel cells using pure hydrogen. Hydrogen production is commercially proven, but expensive. One method of producing hydrogen is steam methane reforming where hydrocarbons and water are reacted to form CO and H2, followed by a separate water-gas-shift reaction where CO is reacted with H2O to form CO2 and H2. The commercial application of these reactions in many refineries commonly involves a series of reactors including a steam reforming reactor, and several post reactors to address the production of CO in the reformer. The post reactors include a high temperature shift reactor, a low temperature shift reactor, and a CO2 absorber separator. Water and CO2 separation is necessary to achieve pure hydrogen. The reforming reactor is run at high pressure to avoid hydrogen recompression downstream. The pressure lowers the equilibrium conversion since reforming produces a positive net mole change. The steam reforming reaction is very endothermic, about 206 kJ/mole; and the shift reaction is exothermic, providing about 41 kJ/mole. The conventional steam reforming reactors are operated above 900xc2x0 C. to push the equilibrium toward complete formation of CO and H2. The high temperature causes severe corrosion and stress problems on the equipment. Steam reforming reactors are generally large to accomplish economies of scale. In addition, the typical operation of the shift reactor at a lower temperature than the reforming reactor makes it impractical to combine these two chemical reactions in one reactor. Furthermore, designs currently known do not lend themselves to being scaled down to a smaller size or to making it possible to efficiently control the temperature at various points.
Even if a reactor was capable of producing only CO2 and H2 and the conventional post reactors could be eliminated, the issue of CO2 separation would remain.
In experimental work the use of membranes to harvest hydrogen from a reforming process is known. For example, U.S. Pat. No. 4,810,485 discloses a hydrogen forming process which comprises conducting in a hydrogen production zone a chemical reaction forming mixed gases including molecular hydrogen, contacting one side of a hydrogen ion porous and molecular gas nonporous metallic foil with said mixed gases in said hydrogen production zone, dissociating said molecular hydrogen to ionic hydrogen on said one side of said metallic foil, passing said ionic hydrogen through said metallic foil to its other side, and withdrawing hydrogen from said other side of said metallic foil, thereby removing hydrogen from said hydrogen production zone. This process takes place at a temperature of from about 1000xc2x0 F. to 1400xc2x0 F.
U.S. Pat. No. 5,525,322 discloses a process for the simultaneous recovery of hydrogen and hydrogen isotopes from water and from hydrocarbons which comprises mixing carbon monoxide and water with the feed mixture forming a gas mixture such that the reversible reactions CO+H2O ⇄CO2+H2 and CH4+H2O⇄CO+3H2 can occur, flowing the gas mixture over a heated nickel catalyst such that the equilibrium of the reactions permits subsequent generation of H isotopes, contacting the resulting gas mixture with a heated palladium membrane, and removing the H isotopes which have permeated the Pd membrane. The reactor is heated by enclosing it in a split-hinge tube furnace.
U.S. Pat. No. 5,741,474 discloses a process for producing high-purity hydrogen which includes heating a reforming chamber provided with a hydrogen-separating membrane, feeding into the reforming chamber hydrocarbon, steam, and oxygen or air to give rise to steam reforming and partial oxidation therein to produce a reaction gas, and passing the reaction gas through the hydrogen-separating membrane to recover high-purity hydrogen. The heat possessed by the portion of the reaction gas not permeable into the hydrogen-separating membrane and the heat generated by the partial oxidation are utilized for the heating and reforming of the hydrocarbon, water and oxygen or air.
U.S. Pat. No. 5,861,137 discloses a compact, mobile steam reformer that includes a tubular hydrogen permeable and hydrogen selective membrane. A reforming bed surrounds at least part of the membrane. An inlet to the reforming bed receives a mixture of alcohol or hydrocarbon vapor and steam and an outlet from the reforming bed releases reforming byproduct gases. A heating element heats the reforming bed to an operating temperature and a second bed including a methanation catalyst is placed at the permeate side of the membrane. A reformer outlet withdraws hydrogen gas from the second bed. In one aspect, the heating element is a third bed including an oxidation catalyst surrounding at least a portion of the first bed. The reforming byproduct gases released from the reforming bed mix with an air source and catalytically ignite to generate heat and thermally support the process of reforming within the reforming bed.
U.S. Pat. No. 5,229,102 discloses a steam reforming process that does not require a shift reactor. It requires a gas turbine to produce hot exhaust gases. That reference discloses a process employing the steps of:
a) providing a generally tubular, porous, ceramic membrane, and providing a heated reaction zone in a container into which the membrane is received,
b) wherein the membrane carries a catalytically active metallic substance,
c) passing a hydrocarbon and steam containing first fluid stream into the reaction zone and into contact with one side of the membrane to produce CO2 and H2,
d) and passing a stream containing second fluid stream adjacent the opposite side of the membrane in such manner as to promote hydrogen diffusion through the membrane from said one side to said opposite side thereof,
e) and removing hydrogen from the opposite side of the membrane.
This process takes place at lower temperatures than are typical of conventional reforming, i.e. 300-750xc2x0 C., however it requires a gas turbine or gas engine to produce hot exhaust gas and the generated heat is transferred into the reaction zone to maintain the temperature.
U.S. Pat. No. 5,938,800 discloses a compact hydrogen generation system that comprises a fuel means for supplying a pressurized, vaporized fuel and steam mixture, a steam reformer having a catalyst bed in communication with the fuel means, and hydrogen filtration means for filtering and removing hydrogen produced in the catalyst bed from the fuel and steam mixture and means for providing same to a collection header, and burner means integrated with the steam reformer for providing hot flue gases to heat the catalyst bed and to make the vaporized fuel and steam mixture by combustion of a least one of an off-gas produced by the steam reformer and an auxiliary fuel, whereby the steam reformer, fuel means and burner means are mobile and lightweight.
It would be desirable in the art to provide a steam reformer reactor design for producing hydrogen completely free of carbon and carbon oxides and with minimal production of NOx. If the pure hydrogen produced could be used to create power in a hybrid system that could be compact in design and provide 71% or greater efficiency in the production of energy it would represent a distinct advance in the art. In addition, it would be desirable if lower temperatures could be used and if the entire process permitted more control over temperatures at various points, or load-following capabilities. Furthermore, if the process produced CO2 in higher concentrations and greater purity than other processes in the art, and the CO2 could be sequestered for other uses, it would be extremely desirable. Such an integrated system would demonstrate far greater efficiency than any power generating system currently available.
In accordance with the foregoing, the present invention accomplishes these objectives and is a new process and apparatus for steam reforming of any vaporizable hydrocarbon to produce H2 and CO2, with minimal CO, and virtually no CO in the H2 stream, said process being accomplished in one reactor, at lower temperatures than those used in conventional stream methane reforming reactors, constantly removing pure hydrogen, and using as a heat source flameless distributed combustion which provides great improvements in heat exchange efficiency and load-following capabilities to drive the steam reforming reaction. Similar efficiency and load-following is simply not possible with conventional firebox steam reformer furnace designs and multi-reactor shift units. The flameless distributed combustion heat source makes it possible to transfer between 90 and 95% of the heat to the reacting fluids. In another embodiment, the invention is also a zero emission hybrid power system wherein the produced hydrogen is used to power a high-pressure internally or externally manifolded molten carbonate fuel cell. The system is capable of achieving 71% or greater efficiency in the conversion of fuel to electricity. In addition, the design of this flameless distributed combustion-membrane steam reforming reactor (FDC-MSR) fueled hybrid system makes it possible to capture high concentrations of CO2 for sequestration or use in other processes. Finally, the design of the system can be scaled down to a mobile, lightweight unit.
The process for steam reforming of any vaporizable hydrocarbon to produce purified H2 and CO2 comprises:
a) Providing a generally tubular reforming chamber having one or more inlets for vaporizable hydrocarbons and steam and one or more corresponding outlets for by-product gases, including H2O, and CO2, with a flow path in between said inlet and outlet, and having one or more inlets for sweep gas (which may be H2O in the form of steam, or other gas such as recycled CO2, nitrogen or condensable hydrocarbons) and corresponding outlets for the sweep gas and hydrogen, with a flow path between said inlet and outlet, and having one or more inlets for preheated air and corresponding inlets for fuel gas mixtures, with a flow path between said inlets containing a plurality of flameless distributed combustion heaters,
wherein said flow path for vaporizable hydrocarbon and flow path for sweep gas form two concentric sections with an annulus between having a reforming catalyst therein;
b) Feeding a vaporizable hydrocarbon and steam into said reforming chamber through said inlet for a vaporizable hydrocarbon and steam;
c) Flowing said vaporizable hydrocarbon over a reforming catalyst;
d) Causing both steam reforming and the shift reaction to take place in said reforming chamber; and
e)Conducting said reforming in the vicinity of a hydrogen permeable and hydrogen-selective membrane, whereby pure hydrogen permeates said membrane;
f) Wherein heat to drive said reaction is provided by said flameless distributed combustors.
The process of the present invention may also be described as a process for the production of hydrogen, comprising:
a) reacting steam with a vaporizable hydrocarbon at a temperature of from about 200xc2x0 C. to about 700xc2x0 C. and at a pressure of from about 1 bar to about 200 bar in a reaction zone containing reforming catalyst to produce a mixture of primarily hydrogen and carbon dioxide, with a lesser amount of carbon monoxide;
b) providing heat to said reaction zone by employing flameless distributed combustion thereby driving said reaction;
c) conducting said reaction in the vicinity of a hydrogen-permeable and hydrogen-selective membrane, whereby hydrogen formed in said reaction zone permeates through said selective membrane and is separated from said carbon dioxide and carbon monoxide.
In order to produce electricity with zero emissions and capture CO2, the pure hydrogen which permeates the membrane may be directed to the anode of a high pressure molten carbonate fuel cell and the by-products from the reforming reaction are directed to the cathode of said fuel cell. The high purity hydrogen may also be directed to other types of fuel cells such as PEM (proton exchange membrane) fuel cells or SOFC (solid oxide fuel cells) and the like.
The invention also pertains to an apparatus comprising a membrane steam reformer heated by flameless distributed combustion to produce high purity hydrogen that may be used for a variety of purposes including as fuel to a high pressure molten carbonate fuel cell or a PEM fuel cell. The integrated flameless distributed combustion-membrane steam reforming reactor (FDC-MSR) of the present invention comprises:
A reforming chamber comprising a generally tubular reactor having two concentric sections comprising a larger-outside section and a smaller inside section and an annulus between said sections, wherein said outside section has an inlet for preheated air and a corresponding inlet for fuel gas, with a flow path between and a plurality of flameless distributed combustors arranged in a circular path in said outside section; and wherein said inside section has an inlet for sweep gas and an outlet for sweep gas and H2, and said annulus has an inlet for vaporizable hydrocarbons and an outlet for by-product compounds and a hydrogen-selective, hydrogen-permeable membrane positioned either on the inside or outside of the annular section. In a further embodiment of the invention said reforming chamber is in communication with a high pressure molten carbonate fuel cell, wherein the outlet for hydrogen from the reformer is in communication with the anode of said fuel cell and the outlet for by-product compounds is in communication with the cathode of said fuel cell.
The integrated FDC-MSR process and apparatus of this invention is capable of producing high purity hydrogen with minimal production of CO and virtually no CO in the hydrogen stream. By practice of the invention it is possible to produce hydrogen having a high purity, e.g., a purity on a dry basis of greater than 95%. The present invention can be used to produce hydrogen having purities as high as 97%, 99%, or under optimum conditions 99+%. The effluent (by product) stream from the MSR reactor will typically contain more than about 80% CO2 on a dry basis, e.g., 90% CO2, 95% CO2 or 99% CO2, and less than about 10% CO on a dry basis, e.g., less than about 5% CO, preferably less than 1% CO
Total heat management and turbines may be included in the system to increase the efficiency and produce additional electricity or to do useful work such as to compress gases or vapors.